This study aimed at investigating the effect of illicit drugs abuse on the academic performance of students in Kajiado North Sub-County. The objectives of the study were to; determine the commonly abused illicit drugs, establish the causes of illicit drugs abuse among students, determine the effects of illicit drugs abuse on academic performance and establish measures put in place to curb illicit drugs abuse in school. The study employed descriptive survey research design. Target population was 32 secondary schools in Kajiado North Sub-County. Simple random sampling technique was used to sample 11 secondary schools. Twelve form four students were sampled using class register from each of the 11 schools sampled. Eleven principals and 11 guidance and counseling teachers from each sampled school were also used as respondents. A total sample size of 154 respondents was used. The study used questionnaires to collect primary data. The data collected was analyzed using Statistical Package for Social Sciences (SPSS) software program and presented using descriptive methods such as frequencies, means, standard deviation and percentages and presented as tables and figures. The study established that alcohol, bhang and tobacco were the most abused illicit drugs in secondary schools in Kajiado North Sub-County. Sources of drugsidentified were fellowstudents and small kiosksor shops in the schools’ neighbourhoods. Causes of illicit drugs abuse established were; peer influence, availability of money to buy illicit drugs and the family background. The study concluded that illicit drugs abuse among the students led to school drop outs, strained relationship with other students, lack of interest in studying, truancy, low concentration span and healthy problems such as anxiety, headaches, feeling sleepy and confusion leading to declining academic performance. Based on these findings, the researcher recommended that schools should develop an illicit drug policy, to create awareness of the effect of illicit drugs abuse among students and liase with all educational stakeholders so as to improve students’ academicperformance.Studentswhoareidentifiedas illicit drug abusers should not be expelled from schools but should be accepted in the community and be helped through guidance and counselling. Although the Ministry of Education has integrated illicit drugs abuse in some subjects, it should also consider developing a curriculum on effects of illicit drugs abuse. This will ensure that all students are informed about consequences of illicit drugs abuse in order to make informed judgments. The study suggests that further studies should be carried out on involvement of teachers and parents in curbing illicit drug abuse and students perception on strategies used by school principals.